The invention relates generally to optics, and more particularly to multilayer optic devices and methods for making the same.
Numerous applications exist that require a focused beam of electromagnetic radiation. For example, energy dispersive X-ray diffraction (EDXRD) may be used to inspect checked airline baggage for the detection of explosive threats or other contraband. Such EDXRD systems may suffer from high false positives due to weak diffracted X-ray signals. The weakness of the X-ray signals may stem from a variety of origins. First, the polychromatic X-ray spectrum used in EDXRD is produced by the Bremsstrahlung part of the source spectrum, which is inherently low in intensity. Second, X-ray source collimation may eliminate more than 99.99 percent of the source X-rays incident on the baggage volume under analysis. Third, some of the materials being searched for, e.g., explosives, may not diffract strongly as they are amorphous. Fourth, the diffracting volume may be small. The last two limitations arise from the type of threat materials being searched for in baggage, making all but the second limitation unavoidable. Although discussed in the context of explosives detection, the limitations described above are equally applicable to medical situations.
At lower X-ray energies, such as 80 keV and below, increasing the polychromatic X-ray flux density at the material being inspected has been addressed by coupling hollow glass polycapillary optics to low powered, sealed tube (stationary anode) X-ray sources. An example of hollow glass polycapillary optics may be found in, for example, U.S. Pat. No. 5,192,869. The glass is the low index of refraction material, and air filling the hollow portions is the high index of refraction material. These types of optics typically do not provide much gain at energy levels above 80 keV, since the difference in the indices of refraction between air and glass becomes increasingly small as energy levels approach and surpass 80 keV.
Further, such optics use a concept of total internal reflection to reflect X-rays entering the hollow glass capillaries at appropriate angles back into the hollow capillaries, thereby channeling a solid angle of the source X-rays into collimated or focused beams at the output of the optic. As used herein, the term “collimate” refers to altering the divergence of beams of electromagnetic (EM) radiation from the intrinsically divergent EM beams. Only about five percent of an EM source's solid angle typically is captured by the input of such known optics.
In addition, the use of air in known optics as one of the materials prevents such optics from being placed within a vacuum. Thus, known optics are limited in their potential uses.
The shaping of an X-ray spectrum to optimize it for particular applications is a common procedure. The change in the spectral shape, for example, reducing either the relative proportion of low-energy X-rays or the relative proportion of high-energy X-rays, can in some cases provide for optimum imaging of a sample. One common artifact in radiographic and tomographic imaging arises from the fact that the lower energy X-rays in a typical Bremsstrahlung (polychromatic) spectrum are attenuated preferentially as the beam penetrates material. This effect, which leads to an increase in the mean energy of the beam as it penetrates the sample, introduces a biasing in the relationship between the strength of the transmitted beam and the amount of material penetrated. This biasing manifests as artifacts in any images reconstructed from the attenuation data, such as those attributed to beam hardening in computed tomography. Utilizing an X-ray beam that has a reduced spread of energies can mitigate some of these artifacts. Particularly where beam intensity, with respect to the intensity in that same range of the original spectrum, has been held constant or augmented by the use of the optic, the use of a limited range of energies can provide a desired degree of attenuation for a particular application and can produce an optimum image in terms of spatial resolution and contrast sensitivity. The shaping of a spectrum from a polychromatic energy distribution to a more monochromatic distribution can enable such improvements in X-ray image sets.
Spectral imaging also includes a single energy distribution as well as multi-energy distributions. Multi-energy X-ray imaging, sometimes referred to as dual-energy imaging or energy discrimination imaging, has been shown to furnish information on specific material compositions in scanned objects for security, industrial, and medical applications. Such energy discrimination imaging can be achieved in several ways, including the use of two or more different X-ray spectra, which is often the most feasible approach. A challenge lies in the sequential nature of such an examination, where image data are generated, for example, first with one spectrum and then with another spectrum. In one technique, an object of interest is scanned twice. A first complete projection data set is produced in the first scan for one energy and then a second complete projection data set is produced in the second scan for the second energy. For many applications where high throughput is critical, sample composition is dynamic, and/or sample positioning may preclude repetitive scanning, the logistics of physically scanning an object twice may be unacceptable.
Conventional multi-energy X-ray imaging applications have used source filtration and/or high voltage modulation for rapidly altering the spectral characteristics on a time scale comparable to the view-by-view sampling time in a typical CT scan. Such filtration consists of rapidly and sequentially inserting filters of appropriate composition to preferentially attenuate relatively low X-ray energies. Such methodologies are limited in the degree to which attenuation can produce cleanly separated energy intervals, severely restricting the sensitivity of this approach for analyzing different materials. High voltage modulation to produce different spectral characteristics also has been implemented in some cases with limited success. There is a challenge in both approaches to mitigate registration differences in the image reconstruction projections that result from sample movement between data sets acquired at different energies, as well as a slight misalignment of the X-ray paths that traverse the object, as is incurred with modulating the X-ray beam on a sub-view basis.